Onagawa plant 'remarkably undamaged,' says IAEA

10 August 2012

The Onagawa nuclear power plant on Japan's northeastern coast - the closest plant to the epicentre of the massive earthquake and tsunami of 11 March 2011 - suffered remarkably little damage, a mission from the International Atomic Energy Agency (IAEA) has concluded.

The Onagawa plant (Image: Tohoku)

The 19-strong mission - comprising IAEA staff, members of national regulators and external experts - has delivered its initial report following a two-week visit to Japan, which included a visit to the three-unit plant in Miyagi Prefecture. The team's objective was to observe how different structures, systems and components of the plant responded to the earthquake.

All three boiling water reactors at Onagawa automatically shut down, as designed, when the earthquake occurred. Onagawa 1 briefly suffered a fire in the non-nuclear turbine building. They have since remained offline. The plant sustained far less damage than expected, considering the magnitude of the earthquake and the height of the subsequent tsunami. A major contributing factor to this is that the plant sits on an elevated embankment almost 14 metres above sea level. Although the earthquake knocked out four of the five external power lines, the remaining line provided sufficient power for the plant's three reactors to be brought to cold shutdown.

The team, led by Sujit Samaddar, the head of IAEA's International Seismic Safety Centre (ISSC), held discussions with the plant operators, reviewed log books and repair reports documented after the earthquake, as well as carrying out a visual investigation of the site.

In its draft report, the team said, "The plant experienced very high levels of ground shaking - among the strongest of any plant affected by the earthquake - and some flooding from the tsunami that followed, but was able to shut down safely." However, it noted, "The structural elements of the nuclear power station were remarkably undamaged given the magnitude of ground motion experienced and the duration and size of this great earthquake."

While presenting information collected by the team to the Japanese government, the IAEA recommended that follow-up missions be conducted at Onagawa and reviews be conducted at other Japanese nuclear power plants that have experienced varying magnitudes of earthquake.

The IAEA said that the mission's findings will be added to a database being compiled by the ISSC to provide knowledge for member states about the impact of external hazards on nuclear power plants.

"The data we are collecting will make an important contribution to improving safety," Samaddar said. "Information in the database will allow IAEA member states to measure the performance of their nuclear power plants in the face of external hazards. We are also seeking such data from member states of the IAEA other than Japan."

He noted, "This is an initial step in a much longer process. The level of cooperation and frank sharing of information that we received from the staff at Onagawa nuclear power station and its owner, Tohoku Electric Power Company, sets a very good example."